Appearances and disappearances of Triassic marine macroinvertebrates in the Western Caucasus (Southwestern Russia)
Ruban, Dmitry A.
Department of Tourism, Higher School of Business, Southern Federal University, 23-ja linija Street 43, Rostov-na-Donu, 344019, Russia E-mail: ruban-d@mail.ru
Marine macroinvertebrates (144 genera of ammonoids, bivalves, brachiopods, corals, and sponges) are known in the Western Caucasus (southwestern Russia) from all Triassic stages. This regional palaeontological record can be employed for understanding of the per-stage dynamics of appearances and disappearances in the marine realm. The number of taxa appearances rose regionally in the Early Triassic to peak in the Anisian. After significant drop provoked by the abrupt basin deepening in the Ladinian, this number rose again until the Rhaetian, when it declined. The number of disappearances also peaked in the Anisian, dropped in the Ladinian, and then increased gradually until the end of the Triassic. These patterns demonstrate both similarities with and differences from the global record of originations and extinctions of marine fossils. For instance, the regional Olenekian–Anisian radiation is also traced globally, but it was longer and culminated in the only Carnian on the planetary scale. The relatively low number of appearances in the Induan is interpreted as a long-term effect of the end-Permian mass extinction. As for the end-Triassic mass extinction, it might have been responsible for the relatively high number of the regional Rhaetian disappearances of genera of marine macroinvertebrates, although these disappearances did not occur in the very end of the Rhaetian (the upper Rhaetian deposits are absent in the Western Caucasus). It appears that the different absolute (in Ma) duration of the Triassic stages may influence on the conclusions based on the per-stage analysis of the diversity dynamics (e.g., its consideration makes the strength of the Norian radiation questionable). In contrast, sampling and preservation biases of the regional palaeontological record do not appear to be very large.
Keywords: marine macroinvertebrates, taxonomic diversity dynamics, mass extinction, Triassic, Western Caucasus